Air separation process employing work expansion of high and low pressure nitrogen



Aprll 2, 1968 E, CIM'LER ET AL 3,375,673

AIR SEPARATION PROCESS EMPLOYING WORK EXPANSION OE HIGH AND Low PRESSURE NITROGEN Filed June 22, 1966 Unite Patented Apr. 2, i968 3,375,673 AIR SEPARATION PRUCESS EMPLOYHNG WORK EXPANSION OF HIGH AND LOW PRESSURE NITROGEN Emil Cimler, Port Washington, and Edward H. Van Baush, Pearl River, NX., assignors to Hydrocarbon Research, Inc., New York, NX., a corporation of New .lerse y Filed .lune 22, 1966, Ser. No. 559,475

6 (Claims. (Cl. 62-13) This invention relates to improvements in the separation of air into its various constituents by liquefaction and fractionation.

The recovery of oxygen vapor and nitrogen vapor from air by liquefaction, using reversing exchangers and double column fractionating equipment, is well known and is indicated in the Jenny Reissue Patent No. 23,463.

It has also been disclosed that the Waste gas, usually nitrogen, contains substantial energy and when expanded with work is available to supply substantial additional refrigeration. The refrigeration is required to offset heat leaks and to ymake up for the hea-t di'erence between incoming air and the end products.

As air liquefaction plants have increased in size, the requirement for low temperature refrigeration materially increases and becomes a substantial factor in the cost of the plant. The use of very high speed -turbo-expanders has proven to be an advantage in reducing the cost and it has been general practice to use either the nitrogen at high pressure or, as more recently suggested, the use of low pressure gases which can be utilized when the initial feed air is compressed to a higher pressure than usual. It `will be apparent that in any expansion step it is necessary to maintain a differential of pressure and in Order to have a sufliciently high pressure to permit expansion to `atmospheric pressure, added energy is required in the first instance.

More recently, it has been found desirable, for commerical purposes, to produce not only high purity oxygen vapors as well as high purity nitrogen vapors with a minimum of Waste gas, but also to produce some yields of either liquid oxygen or liquid nitrogen or both.

In each air liquefaction process there is, of course, a minimum quantity of air which must be worked on to produce the required end products and various suggestions have been made for limiting energy input to establish the necessary low temperature level. It does not usually pay to increase compression energy merely `to obtain expansion work.

Our invention is based on the concept of a flexible air liquefaction system wherein a small percentage, usually from 2 to 3 percent, of the incoming yair may be made available as liquid oxygen or liquid nitrogen, or combinations thereof, and at the same time yield high purity oxygen vapor as well as relatively high purity nitrogen vapor.

Our invention is a process arrangement of heat exchangers, gas expanders, and fractionation columns that provide most economical yields of desired end products.

A particular feature of our invention is the use, in combination with the high pressure gas turbo-expander which supplies 'the majority of the refrigeration in the unit, of a second gas turbo-expander operating under a relatively low expansion ratio but supplying extra refrigeration at a very nominal increased cost.

Further objects and advantages of this invention will appear from the following description of a preferred form of embodiment thereof when taken in connection with the attached drawing illustrative thereof, said drawing being a schematic flou/sheet of the air separation plant.

Atmospheric air at 10 which has been filtered, compressed to approximately 140 p.s.i.a., and cooled to approximately F., ows to the reversing exchanger system through reversing valve 14, warm reversing exchanger 16, cold reversing exchanger 18 and by line 20 (or 20a) through check valve 21 (or 21a), emerging at a temperature of about 265 F. The air at about its dewpoint then flows in line 22 to the bottom of the high pressure tower 24.

The primary air fractionation, in the presence of suitable reux, takes place in this tower and high purity nitrogen vapor is withdrawn at 26. This vapor is then passed by line 26a, through reversing exchanger 18 wherein it is heated and then, under control of valve 28, is expanded in turbo-expander 30 wherein work is done. This high purity nitrogen at a temperature of approximately 235 F. in line 32 may then be used to reboil the bottoms of the high pressure tower 24, in exchanger 34. The cold nitrogen in line 36, now at about 265 F., is then brought to ambient temperature by passing through reversing exchangers 18 and 16 in heat exchange with the incoming air. Vapor nitrogen discharges at 3S as a product.

An important feature of the invention is the expansion of the nitrogen in ythe warm turbo-expander 30 as, for example, from a temperature of 120 F. to a temperature of 235 F. with a temperature difference of F. This is compared to a normal expansion in Va cold expander of from about 240 F. to about 295 F. or 300 F. and thus with a temperature diierence of but 55 to 60 F.; the difference in heat energy is nearly double.

Liquid bottoms in line 40 from the high pressure tower 24 containing approximately 35 oxygen is partially vaporized by passage through line 42 and exchanger 34 as previously described. The net oxygen rich liquid 40 from the high pressure tower is subcooled in parallel exchangers 44a and 44b, passed by line 46 to the acetylene absorbers 48, let down in pressure to approximately 30 p.s.i.a. by reducing valve 50, passed through the nitrogen reflux exchanger 52, and finally fed by line 54 to an intermediate point in a low pressure tower 56.

Low purity nitrogen liquid containing approxima-tely 4% to 8% oxygen, is withdrawn at 60 `from the high pressure tower 24, is subcooled in exchanger 52., let down in pressure by reducing valve 62 to approximately 30 p.s.i.a. and fed to vthe top of the low pressure tower 56.

Oxygen vapor is withdrawn at 64 from the lower part of the low pressure tower 56 just above the reboiler 66, and is then warmed up to ambient temperature by passing through reversing exchangers 18 and 1.6 in heat exchange with entering air and ultimately discharges as la product at 68.

Waste gas containing approximately 4% to 8% oxygen is withdrawn overhead at 70 from the top of the low pressure tower 56 at a pres-sure of approximately 30 p.s.i.a. and is passed through exchanger 44a where it is preheated to approximately 290 F. and then by line 72 is expanded doing work in turbo-expander 74 to a pressure of about 20 p.s.i.a. and a temperature of 305 F. This cold waste gas in line 7.6 then flows through exchanger 4d!) and then by line 7-8 to the reversing exchanger system and reversing valves 21 (or 21a). The waste gas then ows through line 20 (or 20a) exchangers 18 and 16 also in heat exchange with entering 'air and through reversing valve 14 and dinally to atmosphere at 80. In the reversing exchangers, the air and waste gas only, reverse in the well known manner in Order to reject water and carbon dioxide from the plant.

It will be apparent that the `disclosed process utilizes the expanded waste gas to sub-cool liquid rich air in exchangers 44a and 44b. This results from the expansion 3 in the turbo-expander of gas already at 290 F. whereby a gas of 310 F. is developed. The use of the dual exchangers 44a and 44h make possible, therefore, the cooling of the oxygen rich gas stream 40 to a temperature of 283 F. which, after passing through absorber 48, enters the low pressure tower S6.

In a typical plant, turbo-expander 30 will expand approximately 15% of the air from a pressure of about 131 p.s.i.a. to about 20 p.s.i.a and will account for from about 70 to 80% of the refrigeration requirements. Turbo-expander 74 will expand about 60% of the air from a pressure of about 33.5 p.s.i.a. to about 20 p.s.i.a. This will produce from 20 to 30% of the refrigeration.

The expanders 30 and 74 provide the refrigeration to maintain the plant in normal heat balance and, in addition, provide refrigeration to permit liquid withdrawal from the plant. .Liquid oxygen may -be withdrawn at 82 from the bottom of reboiler 66, subcooled in exchanger 84 and discharged as product. Liquid nitrogen may be withdrawn at 86 from just below reboiler 66 and also used as product.

A primary air separation plant such as this normally produces a minimum of nitrogen vapor product and maximum vapor oxygen product of high purities. If it is desired to produce greater ratios of nitrogen to oxygen vapor, or a limited quantity of liquid product, either oxygen, nitrogen or argon, additional refrigeration may be required.

For example, when nitrogen vapor product is desired in large quantity, in lieu of oxygen vapor product, a greater amount of nitrogen can be withdrawn in line 26 from the tower 24 by adjusting valve 88 and passed through line 26h for supplemental expansion in the turboexpander 30. A greater amount of expansion may also be accomplished by opening valve 28 or increasing the opening of the nozzles in expander 30 which will provide extra refrigeration. The larger volume of cold gas is then passed in heat exchange at 34 with tower bottoms.

If desired, a part of the nitrogen reflux in stream 60 may be passed lby line 60a through exchanger 84 in the liquid oxygen line 82 to maintain its low temperature or to subcool it.

If the amount of vapors condensed in reboiler 66 is reduced due to the withdrawal of more nitrogen in line 26, less oxygen will be generated at a given purity since a definite relationship exists between reboiler duty and oxygen production. In a case such as this, 31% of the air as nitrogen can be expanded and delivered at line 3S as product with a simultaneous production of a limited amount of oxygen vapor. Approximately 1.5 to 2.0% of the Iair can be delivered also, as a purified liquid product representing a combination of oxygen, nitrogen and argon.

TABLE F PRODUCTS-TYPICAL BASIS 100 T/D AIR FEED TO PLANT TONS PER DAY Oxygen Vapor 99.6% Oxygen Liquid Nitrogen Vapor,

Purity ppm. Oz

Oxygen vapor available at atmospheres pressure. Oxygen liquid `available at normal boling point. Nitroeen vapor available at 1 atmosphere pressure,

Nitrogen liquid available at normal boiling point.

(6) Both expanders operate in a safe vapor region with no danger of carbon dioxide fouling.

Initial air pressure could be higher, i.e., 150 p.s.i.a. without improving the process. Also, ifthe ai-r could be cooler than F. this would be beneficial as less heat would have to be removed. Gener-ally, however, the use of p.s.i.a. air at 100 F. will Ibe sufficient for understanding of the invention Iby one skilled in the art.

While We have shown and described preferred forms;

of embodiment of our invention, we are aware that modifications may 'be made thereto and we, therefore, desire a broad interpretation of our invention within the scope and spirit of the description herein and of the claims appended hereinafter.

We claim:

1. In the method of producing oxygen at an elevated pressure from feed air lat a substantial superatmospheric pressure wherein the air is cooled in reversing exchangers which are periodically purged of carbon dioxide and moisture, to a temperature at which a minor portion thereof is liquefied, and the resulting cold air is passed to the base of the first column of a double fractionating column,

said first col-umn Ibeing maintained at a substantial superatmospheric pressure, and wherein oxygen-enriched liquid air from the base of said first column is withdrawn and expanded into an intermediate point of the second column of said double column, said second column being maintained at a superatmospheric pressure but less than the pressure of the first column, land wherein high pressure nitrogen is removed from the top of said first column; the improvement which comprises supplying refrigeration to the feed air by passing said high pressure nitrogen in indirect heat exchange relationship with the feed air and thereafter expanding said high pressure nitrogen with work through a high speed turbine to a slightly superatmospheric pressure and removing said expanded nitrogen as a first vapor product, withdrawing gaseous oxygen product at the pressure of said second column from the lower end of said second column, passing said withdrawn gaseous oxygen and said expanded gaseous high pressure nitrogen product in indirect heat exchange relationship with said feed air to cool said feed air in said air cooling step, and thereafter collecting said gaseous oxygen product at ambient temperature and at said elevated pressure as a second vapor product, removing a nitro gen rich vapor from the top of the second column, expanding said nitrogen rich vapor from the second column thro-ugh a second high. speed turbine doing work, passing said expanded nitrogen rich vapor from the second column in indirect heat exchange with the oxygen enriched liquid air to cool said oxygen enriched liquid air, and removing said expanded nitrogen rich vapor from the second column as waste.

2. In the method of producing oxygen as claimed in claim 1, the further step of removing liquid oxygen at substantially atmospheric pressure from the lower part of the second column.

3. In the method of producing oxygen. as claimed in claim 1, the further step of removing liquid nitrogen from the upper part of the first column.

4. In the method of producing oxygen as claimed in claim 1, the refrigeration capacity of the first expander being in the range of 70-80% of the total requirements of the plant.

5. In the method of producing oxygen as claimed in claim 1, the refrigeration capacity `of the second expander being in the range of 20-30% of the total requirements of the plant.

6. The method of separating air into its principal coni stituents which comprises passing said air through a series of reversing exchangers in heat exchange with a relatively cold waste gas, vapor oxygen product and vapor nitrogen product to reduce the temperature of the air to substantially its temperature of liquefaction, passing said cold air into a high pressure fractionation zone in the presence 5 of reflux to separate an oxygen enriched bottoms stream from a high pressure nitrogen vapor overhead, cooling said oxygen enriched liquid stream and passing said cold oxygen enriched liquid stream to a low pressure fractionation zone in the presence of reflux to produce a high purity oxygen stream and an impure low pressure waste gas overhead, expanding said l10W pressure waste gas overhead in a turbo-expander doing work to produce the cold waste gas for cooling oxygen enriched liquid stream and the incoming air, warming the high pressure nitrogen and thereafter expanding at least a part of said high pressure nitrogen vapor in a turbo-expander doing work to reduce the temperature thereof, and heat exchanging said reduced temperature expanded nitrogen vapor against bottoms of the high pressure fractionation zone to produce a 1iquefied normally gaseous product.

References Cited UNITED STATES PATENTS 2,918,802 12/ 1959 Grunberg 62-38 XR 3,070,966 1/1963 Ruhemann et al. o 62-39 XR 3,209,548 10/ 1965 Grunberg et al. 63-13 XR 3,216,206 11/1965 Kessler 62-38 XR 10 3,217,502 11/1965 Keith 62-39 XR WILBUR L. BASCOMB, JR., Primary Examiner. V. W. PRETKA, Assistant Examiner. 

1. IN THE METHOD OF PRODUCING OXYGEN AT AN ELEVATED PRESSURE FROM FEED AIR AT A SUBSTANTIAL SUPERATMOSPHERIC PRESSURE WHEREIN THE AIR IS COOLED IN REVERSING EXCHANGERS WHICH ARE PERIODICALLY PURGED OF CARBON DIOXIDE AND MOISTURE, TO A TEMPERATURE AT WHICH A MINOR PORTION THEREOF IS LIQUEFIED, AND THE RESULTING COLD AIR IS PASSED TO THE BASE OF THE FIRST COLUMN OF A DOUBLE FRACTIONATING COLUMN, SAID FIRST COLUMN BEING MAINTAINED AT A SUBSTANTIAL SUPERATMOSPHERIC PRESSURE, AND WHEREIN OXYGEN-ENRICHED LIQUID AIR FROM THE BASE OF SAID FIRST COLUMN IS WITHDRAWN AND EXPANDED INTO AN INTERMEDIATE POINT OF THE SECOND COLUMN OF SAID DOUBLE COLUMN, SAID SECOND COLUMN BEING MAINTAINED AT A SUPERATMOSPHERIC PRESSURE BUT LESS THAN THE PRESSURE OF THE FIRST COLUMN, AND WHEREIN HIGH PRESSURE NITROGEN IS REMOVED FROM THE TOP OF SAID FIRST COLUMN; THE IMPROVEMENT WHICH COMPRISES SUPPLYING REFRIGERATION TO THE FEED AIR BY PASSING SAID HIGH PRESSURE NITROGEN IN INDIRECT HEAT EXCHANGE RELATIONSHIP WITH THE FEED AIR AND THEREAFTER EXPANDING SAID HIGH PRESSURE NITROGEN WITH WORK THROUGH A HIGH SPEED TURBINE TO A SLIGHTLY SUPERATMOSPHERIC PRESSURE AND REMOVING SAID EXPANDED NITROGEN AS A FIRST VAPOR PRODUCT, WITHDRAWING GASEOUS OXYGEN PRODUCT AT THE PRESSURE OF SAID SECOND COLUMN FROM THE LOWER END OF SAID SECOND COLUMN, PASSING SAID WITHDRAWN GASEOUS OXYGEN AND SAID EXPANDED GASEOUS HIGH PRESSURE NITROGEN PRODUCT IN INDIRECT HEAT EXCHANGE RELATIONSHIP WITH SAID FEED AIR TO COOL SAID FEED AIR IN SAID AIR COOLING STEP, AND THEREAFTER COLLECTING SAID GASEOUS OXYGEN PRODUCT AT AMBIENT TEMPERATURE AND AT SAID ELEVATED PRESSURE AS A SECOND VAPOR PRODUCT, REMOVING A NITROGEN RICH VAPOR FROM THE TOP OF THE SECOND COLUMN, EXPANDING SAID NITROGEN RICH VAPOR FROM THE SECOND COLUMN THROUGH A SECOND HIGH SPEED TURBINE DOING WORK, PASSING SAID EXPANDED NITROGEN RICH VAPOR FROM THE SECOND COLUMN IN INDIRECT HEAT EXCHANGE WITH THE OXYGEN ENRICHED LIQUID AIR TO COOL SAID OXYGEN ENRICHED LIQUID AIR, AND REMOVING SAID EXPANDED NITROGEN RICH VAPOR FROM THE SECOND COLUMN AS WASTE. 